Researchers observed a sample of pregnant women in California and saw a 3-percent increase in use form 2009 through 2016.
Researchers said reefer gladness among pregnant women under 18, jumped from 9.8% to 19% and from 12.5% to 21.8% for women in the 18 to 24-year-old demographic.
Younger mothers-to-be were more likely to smoke marijuana.
The data come from toxicology reports and a combination of self-reports in which pregnant females say they have used marijuana and positive toxicology reports.
"There is an increased perception of the safety of cannabis use, even in pregnancy, without data to say it's actually safe", said Dr. Torri Metz, an obstetrician at Denver Health Medical Center who specializes in high-risk pregnancies, according to The New York Times.More news: Pakistan Uses Regional Conference to Denounce US Charges
"Of concern" to the researchers was that fact that marijuana use was particularly common among younger pregnant women.
An advantage of the study, he added, was that women not only self-reported marijuana use but also were screened for marijuana - and he thinks the study findings are age-related, as the largest increase in marijuana use was among adolescents and young adults.
Barbara Yankey, a researcher at Georgia State University, told Reuters marijuana use may be on the rise because of the recent legalization of its recreational use "has made people think of the drug as less unsafe, even during pregnancy".
Along with advising against marijuana use, doctors have long advised pregnant women to avoid drinking alcohol during pregnancy.
"Then there's the other group who take a more pragmatic view of pregnancy".